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KLargestInHeap.java
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KLargestInHeap.java
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package epi;
import epi.test_framework.EpiTest;
import epi.test_framework.EpiTestComparator;
import epi.test_framework.GenericTest;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.PriorityQueue;
import java.util.function.BiPredicate;
public class KLargestInHeap {
private static class HeapEntry {
public Integer index;
public Integer value;
public HeapEntry(Integer index, Integer value) {
this.index = index;
this.value = value;
}
}
private static final int DEFAULT_INITIAL_CAPACITY = 16;
@EpiTest(testDataFile = "k_largest_in_heap.tsv")
public static List<Integer> kLargestInBinaryHeap(List<Integer> A, int k) {
if (k <= 0) {
return Collections.emptyList();
}
// Stores the (index, value)-pair in candidateMaxHeap. This heap is
// ordered by the value field.
PriorityQueue<HeapEntry> candidateMaxHeap =
new PriorityQueue<>(DEFAULT_INITIAL_CAPACITY,
(o1, o2) -> Integer.compare(o2.value, o1.value));
candidateMaxHeap.add(new HeapEntry(0, A.get(0)));
List<Integer> result = new ArrayList<>();
for (int i = 0; i < k; ++i) {
Integer candidateIdx = candidateMaxHeap.peek().index;
result.add(candidateMaxHeap.remove().value);
Integer leftChildIdx = 2 * candidateIdx + 1;
if (leftChildIdx < A.size()) {
candidateMaxHeap.add(new HeapEntry(leftChildIdx, A.get(leftChildIdx)));
}
Integer rightChildIdx = 2 * candidateIdx + 2;
if (rightChildIdx < A.size()) {
candidateMaxHeap.add(
new HeapEntry(rightChildIdx, A.get(rightChildIdx)));
}
}
return result;
}
@EpiTestComparator
public static BiPredicate<List<Integer>, List<Integer>> comp =
(expected, result) -> {
if (result == null) {
return false;
}
Collections.sort(expected);
Collections.sort(result);
return expected.equals(result);
};
public static void main(String[] args) {
System.exit(
GenericTest
.runFromAnnotations(args, "KLargestInHeap.java",
new Object() {}.getClass().getEnclosingClass())
.ordinal());
}
}